The invention relates to a belt retractor comprising a belt reel, a housing part and a signal transmitter ring.
Hereinafter the invention shall be described in the context of belt retractors incorporated in vehicles. Said belt retractors may require to sense the movement of rotation of a belt reel, which is frequently carried out by means of magnetic sensors. A signal transmitter ring according to the invention may also be used in any other applications, however, in which a movement of rotation of a shaft is to be sensed.
In particular applications, especially when the belt retractor includes a reversible tensioning unit, it may be necessary, however, to directly sense the movement of rotation of the reel, for example in order to be able to detect accelerations, rotational speeds or tensioning paths during tensioning. Such sensing is frequently performed by a magnetic disk which co-rotates with the shaft and rotation of which is detected by means of a magnetic sensor arranged on the housing.
In known belt retractors, axial tolerances regarding the position of the retractor shaft of the belt reel cannot be avoided. However, such tolerances are critical to the magnetic sensor, as the latter is intended to reliably provide a measuring signal in all tolerance situations. In this way, very strong (and thus expensive) magnets have to be used which reliably permit positioning even in the case of maximum distance from the sensor.